KILL
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
kill - send signal to a process
SYNOPSIS
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
kill():
_POSIX_C_SOURCE
DESCRIPTION
The
kill()
system call
can be used to send any signal to any process group or process.
If pid is positive, then signal sig is sent to the
process with the ID specified by pid.
If pid equals 0, then sig is sent to every process in the
process group of the calling process.
If pid equals -1, then sig is sent to every process
for which the calling process has permission to send signals,
except for process 1 (init), but see below.
If pid is less than -1, then sig is sent to every process
in the process group whose ID is -pid.
If sig is 0, then no signal is sent,
but existence and permission checks are still performed;
this can be used to check for the existence of a process ID or
process group ID that the caller is permitted to signal.
For a process to have permission to send a signal,
it must either be privileged (under Linux: have the
CAP_KILL
capability in the user namespace of the target process),
or the real or effective user ID of the sending process must equal
the real or saved set-user-ID of the target process.
In the case of
SIGCONT,
it suffices when the sending and receiving
processes belong to the same session.
(Historically, the rules were different; see NOTES.)
RETURN VALUE
On success (at least one signal was sent), zero is returned.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
- EINVAL
-
An invalid signal was specified.
- EPERM
-
The process does not have permission to send the signal
to any of the target processes.
- ESRCH
-
The process or process group does not exist.
Note that an existing process might be a zombie,
a process that has terminated execution, but
has not yet been
wait(2)ed
for.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTES
The only signals that can be sent to process ID 1, the
init
process, are those for which
init
has explicitly installed signal handlers.
This is done to assure the
system is not brought down accidentally.
POSIX.1 requires that kill(-1,sig) send sig
to all processes that the calling process may send signals to,
except possibly for some implementation-defined system processes.
Linux allows a process to signal itself, but on Linux the call
kill(-1,sig) does not signal the calling process.
POSIX.1 requires that if a process sends a signal to itself,
and the sending thread does not have the signal blocked,
and no other thread
has it unblocked or is waiting for it in
sigwait(3),
at least one
unblocked signal must be delivered to the sending thread before the
kill()
returns.
Linux notes
Across different kernel versions, Linux has enforced different rules
for the permissions required for an unprivileged process
to send a signal to another process.
In kernels 1.0 to 1.2.2, a signal could be sent if the
effective user ID of the sender matched effective user ID of the target,
or the real user ID of the sender matched the real user ID of the target.
From kernel 1.2.3 until 1.3.77, a signal could be sent if the
effective user ID of the sender matched either the real or effective
user ID of the target.
The current rules, which conform to POSIX.1, were adopted
in kernel 1.3.78.
BUGS
In 2.6 kernels up to and including 2.6.7,
there was a bug that meant that when sending signals to a process group,
kill()
failed with the error
EPERM
if the caller did not have permission to send the signal to
any (rather
than
all) of the members of the process group.
Notwithstanding this error return, the signal was still delivered
to all of the processes for which the caller had permission to signal.
SEE ALSO
kill(1),
_exit(2),
signal(2),
tkill(2),
exit(3),
killpg(3),
sigqueue(3),
capabilities(7),
credentials(7),
signal(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Linux notes
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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